LJUBLJANA, Slovenia: On December 17, 2018, Kyai Haji Yahya Cholil Staquf—General Secretary of Nahdlatul Ulama (NU), the world’s largest Muslim organization and Emissary of Indonesia’s largest Islamic political party (PKB)—addressed the Slovenian nation at the invitation of its former Prime Minister, Janez Janša. The address, delivered at a heavily-attended public forum in the capital Ljubljana, on the topic of “Migration, Terrorism [and] Freedom of Speech,” came three years after a mass influx of largely Muslim migrants flooded through Slovenian territory into Central and Western Europe, triggering continent-wide social and political upheaval; a wave of Islamist terrorism; and the resurgence of populism as a potent political force for the first time since the Second World War.

In his speech—hosted by the Slovenian Democratic (SDS) and Slovenian People’s (SLS) parties—Mr. Staquf warned that among the consequences of this laissez faire approach to immigration is the re-emergence of supremacist ethno-religious identities that many Europeans pride themselves on having left behind, but are now once again threatening European stability, social harmony and the respect for human rights embraced by its political mainstream.

“If Europe is unable to cope with these major migratory flows—if it fails to assimilate migrants within the framework of European culture and civilization—then Europe will become a wilderness [with the breakdown of law and order],” said Mr. Staquf in a four-page interview with the Slovenian magazine Demokracija. “Uncritically accepting migrants from troubled societies, without the capacity to successfully absorb them, naturally leads to their problems becoming your problems,” continued Mr. Staquf.

Chief among these problems is a mental framework—rooted within certain problematic tenets of Islamic orthodoxy—that views Islam as inherently political (i.e., supremacist), giving rise to a “tribal” identity readily weaponized by unscrupulous state and non-state actors. Mr. Staquf said that Indonesians are intimately familiar with this problem, and that “NU religious leaders established PKB to preserve Indonesia as a multi-religious and pluralistic nation state in the face of serious challenges posed by Muslim extremists after the fall of Suharto’s authoritarian regime.”

Slovenia—for centuries on the bloody faultline between the Ottoman Empire and Christian Europe, and the first country to secede from the Socialist Federal Republic of Yugoslavia in 1991, as it collapsed into a series of ethno-religious insurgencies and civil wars—is acutely aware of the dangers posed by ethnic animosity and religious supremacism. Mr. Staquf said that, “Due to its history and strategic location, Slovenia is well-positioned to identify and help tackle these problems not only on a European, but also at a global level.”

Mr. Staquf’s Ljubljana address is part of a wider effort to build the societal consensus necessary to block the political weaponization of Islam in Europe and worldwide. In 2017 the Rotating Presidency of the EU Council introduced policy recommendations by Mr. Staquf via the EU Council Terrorism Working Party to help reconcile the severe policy differences—triggered by mass migration—that have strained relations between Western European nations led by Germany and the V4 (the Czech Republic, Hungary, Poland and Slovakia). Interventions by the NU General Secretary helped shape public discourse amid the heat of Germany’s 2017 national election campaign and helped strip Saudi control of a Brussels terror mosque linked to Islamic State attacks in the Belgian capital as well as Paris.